A molecular fluorophore in citric acid/ethylenediamine carbon dots identified and quantified by multinuclear solid-state nuclear magnetic resonance

Pu Duan, Bo Zhi, Luke Coburn, Christy L. Haynes, Klaus Schmidt-Rohr

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Abstract

The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave-assisted reaction of citric acid and ethylenediamine was investigated by 13C, 13C{1H}, 1H─13C, 13C{14N}, and 15N solid-state nuclear magnetic resonance (NMR) experiments. 13C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13C NMR peaks, including an unusual ═CH signal at 84 ppm (1H chemical shift of 5.8 ppm) and ═CN2 at 155 ppm, and two distinctive 15N NMR resonances near 80 and 160 ppm proved the presence of 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross-peaks in a 1H─13C HETCOR spectrum with brief 1H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13C and 15N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13C chemical-shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation.

Original languageEnglish (US)
Pages (from-to)1130-1138
Number of pages9
JournalMagnetic Resonance in Chemistry
Volume58
Issue number11
DOIs
StatePublished - Nov 1 2020

Bibliographical note

Funding Information:
The authors thank Dr. Q. Cui for his instrumental role in initiating this collaborative project. Contributions to this work by B. Z. and C. L. H. were supported by the National Science Foundation under the Center for Sustainable Nanotechnology, CHE-1503408. The Center for Sustainable Nanotechnology is part of the Centers for Chemical Innovation Program. The solid-state NMR spectrometer used in this work was funded by the National Science Foundation (Award No. 1726346).

Funding Information:
The authors thank Dr. Q. Cui for his instrumental role in initiating this collaborative project. Contributions to this work by B. Z. and C. L. H. were supported by the National Science Foundation under the Center for Sustainable Nanotechnology, CHE‐1503408. The Center for Sustainable Nanotechnology is part of the Centers for Chemical Innovation Program. The solid‐state NMR spectrometer used in this work was funded by the National Science Foundation (Award No. 1726346).

Keywords

  • C{N} NMR
  • Quantitative solid-state C NMR
  • fluorescent carbon dots
  • molecular fluorophores
  • photoluminescent polymer dots
  • quantitative solid-state C NMR
  • spectral editing
  • spectral editing

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